This invention relates to data communication systems and particularly to an interface between a data bus, which may be connected to or from part of a local area network, and a system which operates in an asynchronous transfer mode, which conveys data in virtual channels according to a time shared process which allows a multiplicity of different messages to be transmitted over a single path in a time-shared manner, the messages being in the form of packets composed of cells, there being no regular or prescribed spacing between the cells.